(🐞) for/else with break statement incorrect possibly-undefined error

[KotlinIsland]

#14191 (comment)

a: list[int]

for i in a:
    if i:
        b = i
        break
else:
    b = 1

print(b)  # error: Name "b" may be undefined  [possibly-undefined]

[Dreamsorcerer]

Also when the else condition raises:

x: int
while x:
    if x == 5:
        foo = 10
        break
else:
    raise RuntimeError()

foo  # possibly-undefined

[erictraut]

I'm not able to repro the problem with the latest version of mypy. But then again, I'm not able to repro with any versions going back to 0.990, so maybe I'm not using the right configuration.

@KotlinIsland, are you able to repro this issue with the latest version?

BTW, it would be useful in the future if you included the mypy version in your bug reports as requested in the bug template.

[Dreamsorcerer]

Did you miss enabling the error code? mypy --enable-error-code 'possibly-undefined'

Both still reproduce on 1.5.

[erictraut]

Ah, that was what I was missing. I though this error code was enabled by default in recent versions of mypy.

[KotlinIsland]

@erictraut You should try using basedmypy, all errors are enabled by default.

[Dreamsorcerer]

It also seems impossible to get something like this to type check:

for i in range(5):
    b = True

print(b)  # error: Name "b" may be undefined  [possibly-undefined]

I guess mypy doesn't know how to confirm that the loop is guaranteed to execute.

[giacomo-alzetta-aiven]

This also fails:

for element in sequence:
    if (something := something_else.get(element)) is not None:
        break
else:
    return None
print(something)   # triggers possibly-undefined

Seems like mypy doesn't understand that the else clause of the for loop will run if sequence is empty...

[NMertsch]

I never contributed code to mypy before, but would love to get this fixed.
Is this a "good first issue" candidate?

[KotlinIsland]

Yeah, it would be a relatively simple scenario. Likely only one spot that needs fixing

[NMertsch]

I started my Draft PR (#17720) by adding three failing test cases reproducing the issue. If someone can help me with a builtins/fixture issue, that would be highly appreciated.

It's not a blocker yet (I have two "good" failing tests to work with), but I'm curious to learn more about how this is done "properly" 🙂

[KotlinIsland]

update unit/fixtures/exception.pyi to:

class list:
    def __iter__(self): ...

[NMertsch]

While investigating this issue, I determined five separate issues or possible improvements with possibly-undefined and loops:

1. for and while loop: else execution is dependent on position, not just presence of break (cause for this issue)
2. for and while loop: else is not just "no break", but also executed if the body is not executed
   • comment: (🐞) for/else with break statement incorrect possibly-undefined error #14209 (comment)
   • fix in [possibly-undefined] Fix loop misconceptions #17720
3. for loop: index is only defined if the body is executed
   • fix in [possibly-undefined] Fix loop misconceptions #17720
4. for loop: infer emptiness of expression
   • comment: (🐞) for/else with break statement incorrect possibly-undefined error #14209 (comment)
   • see also: Non-empty container types #9745
   • see also: Possibly-undefined: allow assertions on the lengths of sequences #14830)
5. while loop: infer truthiness of expression
   • comment: (🐞) for/else with break statement incorrect possibly-undefined error #14209 (comment)

I hope I can get #17720 merged, but currently don't have the capacity to work on the other points.